1.Field of the Invention
The present invention relates to a signal processing circuit for processing an electric signal of a sensor, the sensor having a sensor element for electrically detecting a change in physical amount of a workpiece and a signal output means for detecting the change of the sensor element and outputting the electric signal.
2.Description of the Related Art
Conventionally, a sensor having a sensor element for electrically detecting a change in a physical amount of a workpiece and a signal outputting means for detecting the change of the sensor chip and outputting an electric signal is used.
For example, an electro-capacitance type sensor having a diaphragm deforming in proportion to a change in pressure of the workpiece, a substrate for supporting the diaphragm, a sensor element for detecting the deformation of the diaphragm as a change in electro-capacitance between the diaphragm and the substrate and an electro-capacitance signal output means for detecting the change in the sensor element and outputting an electro-capacitance signal is known as such sensor.
Such electro-capacitance sensor is used as a pressure sensor and an acceleration sensor. In the pressure sensor, for example, the deformation of the diaphragm in proportion to the change in the pressure of the workpiece can be electrically detected as a change in the electro-capacitance, which is suitable for controlling and measuring pressure utilizing a computer.
As a specific example, a pressure detector 90 using an electro-capacitance pressure sensor is shown in FIG. 10.
In the figure, the pressure detector 90 has a base member 91 which has a fitting 92 fixedly screwed to a portion to be detected. The fitting 92 is provided with a pressure inlet 93 to which pressure is introduced from an inside of the portion to be detected. The base member 91 has a greatly enlarged diameter remote from the fitting 92 and a pressure sensor 1 is installed thereon to cover the enlarged opening. A sealing member 94 such as an O-ring is inserted between the pressure sensor 1 and the base member 91 in order to ensure sealability therebetween.
The pressure sensor 1 has a diaphragm 11 on a surface facing the pressure inlet 93, the diaphragm 11 receiving the pressure from the pressure inlet 93 to displace in a direction intersecting the surface. The pressure sensor 1 outputs the displacement of the diaphragm 11 as a change in electro-capacitance.
A processor 5 is mounted on the pressure sensor 1 remote from the fitting 92. The processor 5 is connected to an electrode of the pressure sensor 1 through a through-hole etc. provided on the pressure sensor 1 to receive a signal showing the change in the electro-capacitance and to output to the outside after amplifying and conducting predetermined arithmetic processing etc.
An output substrate 95 is mounted for outputting the signal from the processor 5 to the outside. The processor 5 and the output substrate 95 are wired by wire-bonding etc. and a durable cable 96 is wired to connect the output substrate 95 and the outside.
The base member 91 is covered by a cover member 97, in which all of the pressure sensor 1, the processor 5, the output substrate 95 etc. are accommodated.
In the electro-capacitance sensor used in the above-described pressure detector and the like, the actual pressure change and the electro-capacitance change do not linearly correspond within all the measurement range. Moreover, when the electro-capacitance sensors are mass-produced, slight deviation is caused in each product.
Accordingly, a calibration work is conducted in order to measure highly accurately irrespective of measurement range and the electro-capacitance sensor employed, in which respective sensors are adjusted by a potentiometer or a thick-film resistor or a thin-film resistor is trimmed by laser trimming.
However, since such calibration work requires an outside adjusting device as well as the sensor and the calibration work has to be conducted for respective sensors, large cost is necessary for the calibration work after manufacturing the sensors.
In view of above problem, a self-calibrating sensor is proposed (Japanese Patent Publication No. 2676959), in which a signal processing circuit having processor including adjusting device is integrally provided on the electro-capacitance sensor, so that the calibration work is conducted by the sensor itself.
Specifically, the self-calibration method by the processor of the above publication is summarized as follows.
In general, there is a relationship represented by following formula (1) between a sensor voltage output V in proportion to the change in the electro-capacitance of the sensor and pressure P.                     V        =                                            a              xc3x97              P                                      b              -                              P                2                                              +          c                                    (        1        )            
Here, a, b and c are calibration values. Calibration of the sensors is equal to finding proper values for the calibration values a, b and c.
When standard pressures for the calibration are set as P0, P1 and P2, and sensor outputs corresponding thereto are set as V0, V1 and V2, following formulas (2) to (4) can be derived by assigning above values into the formula (1).                               V          0                =                                            a              xc3x97                              P                0                                                    b              -                              P                0                2                                              +          c                                    (        2        )                                          V          1                =                                            a              xc3x97                              P                1                                                    b              -                              P                1                2                                              +          c                                    (        3        )                                          V          2                =                                            a              xc3x97                              P                2                                                    b              -                              P                2                2                                              +          c                                    (        4        )            
The processor solves simultaneous equations of the formulas (2) to (4) to calculate the calibration values a, b and c and the calibration values are stored in a memory such as E2PROM (or EEPROM; Electrically Erasable and Programmable Read Only Memory) provided on the sensor.
In actual measurement, the pressure P is calculated after correcting the detected sensor voltage output V using the calibration values a, b and c calculated by the above formulas (2) to (4). More specifically, in the processor on the sensor, the calibration values a, b and c obtained by the above formulas (2) to (4) are assigned to formula (5) representing solution of the quadratic equation of formula (1), and the detected sensor voltage output V is corrected.                     P        =                              a            ±                                                            a                  2                                +                                  4                  xc3x97                                                            (                                              V                        -                        c                                            )                                        2                                    xc3x97                                      xe2x80x83                                    ⁢                  b                                                                                        -              2                        xc3x97                          (                              V                -                c                            )                                                          (        5        )            
According to the self-calibrating sensor, since the adjustment is conducted by the processor of the signal processing circuit integrally provided on the sensor, no calibration work is necessary for respective sensors using separate adjusting device after manufacturing the electro-capacitance sensor, thereby largely reducing the cost required for the calibration work of the sensors.
However, there are following disadvantages in the above-described self-calibrating sensor.
The above-described self-calibrating sensor calculates the calibration values a, b and c using the simultaneous equations of formulas (2) to (4) and corrects the output by the formula (5). Accordingly, the processor of the signal processing circuit needs to include both calculating section of the calibration values a, b and c by the formula (2) to (4) and output correcting section using the formula (5), resulting in complicated structure of the processor. Moreover, the calibration values calculating section according to the formulas (2) to (4) is not used in the actual pressure measurement after calibration, inevitably resulting in unnecessary parts not used for the actual measurement. Accordingly, even though the cost for the calibration work can be reduced, total cost including the cost for respective components is not necessarily reduced on account of complicated structure of the processor in the sensor.
Similar disadvantages also occur to a self-calibrating sensor, in which physical amount of the workpiece is electrically detected and an electric signal is outputted by detecting the change in a sensor element, such as a strain gauge sensor for detecting a deformation of a diaphragm as a change in a strain gauge, not limited to the above-described electro-capacitance sensor.
An object of the present invention is to provide a signal processing circuit of a sensor for efficiently utilizing the processor without complicating the structure of the processor, thereby reducing total cost of the sensor.
In order to attain the above object, the present invention is characterized in simplifying the structure of the processor by commonizing section for calculating calibration values and section for correcting output.
Specifically, a signal processing circuit of a sensor according to the present invention is for processing an electric signal, the sensor having a sensor element for electrically detecting a change in physical amount of a workpiece and a signal output means for detecting the change of the sensor element and outputting the electric signal. The signal processing circuit is characterized in having a processor for obtaining a reduced value P of the physical amount by calibrating a detected value k obtained by the electric signal with a transforming formula (6) of;                     P        =                              γ            xc3x97                          (                              k                -                β                            )                                            1            +                          α              xc3x97                              (                                  k                  -                  β                                )                                                                        (        6        )            
The processor is further characterized in having; a coefficient arithmetic section for calculating coefficients xcex1, xcex2 and xcex3 of the transforming formula by obtaining a detected value corresponding to known physical amount on optional three points within measurement range of the sensor and assigning the detected value to the transforming formula; a calibration arithmetic section for obtaining the reduced value P of the physical amount by assigning the calculated coefficients xcex1, xcex2 and xcex3 and calibrating the detected value k detected in accordance with unknown measured physical amount by the transforming formula; and a fundamental arithmetic section for conducting calculation represented by the formula (7) of                     f        =                              Z            -            W                                X            -            Y                                              (        7        )            
based on a predetermined arguments X, Y, Z and W inputted by the coefficient arithmetic section or the calibration arithmetic section and outputting a calculation result f to the coefficient arithmetic section or the calibration arithmetic section.
Here, the sensor element refers to element electrically detecting the physical amount of the workpiece. For instance, the sensor element includes a strain gauge sensor element having diaphragm etc. deforming in proportion to the change in physical amount of the workpiece where the deformation of the diaphragm is detected as a change in the resistance value of the strain gauge, an electro-capacitance sensor element detecting as a change in the electro-capacitance, and other sensor element electrically detecting the change in the physical amount of the workpiece using photodiode and the like without deforming the sensor element itself.
All of the above are sensor elements electrically detecting the change in the physical amount of the workpiece, and requiring calibration work for corresponding the reduced value of the physical amount based on the electric signal and the actual physical amount acting on the sensor element, which can be calibrated by the above described transforming formula (6).
The strain gauge sensor element may include a bridge circuit mutually connecting four strain gauges provided on the diaphragm of, for instance, the pressure sensor having diaphragm deforming in proportion to the change in the pressure of the workpiece. In the strain gauge sensor element, the deformation amount of the diaphragm can be detected as an electric signal by applying a predetermined voltage to an end of the bridge circuit and obtaining the change in potential difference of the other end in proportion to the change in resistance value of the strain gauge. The potential difference may be adopted as the detected value k and may be calibrated by the above transforming formula (6).
The electro-capacitance sensor element may include a movable electrode formed on the diaphragm, and first and second fixed electrodes formed on a substrate supporting the diaphragm opposite to the movable electrode. The electro-capacitance sensor element can detect the deformation amount of the diaphragm by obtaining the change in first electro-capacitance signal C1 between the movable electrode and the first fixed electrode and second electro-capacitance signal C2 between the movable electrode and the second fixed electrode.
The above movable electrode may be formed not only along one side of the diaphragm but may be formed on both sides of the diaphragm. When the movable electrode is formed on one side of the diaphragm, the first and the second fixed electrode support the diaphragm and are parallel formed on the substrate opposing the movable electrode. Incidentally, when the movable electrode is formed on both sides of the diaphragm, the first fixed electrode opposes one side of the movable electrode and the second fixed electrode opposes the other side of the movable electrode, thereby forming differential pressure sensor for detecting differential pressure etc. of a space partitioned by the diaphragm.
The detected value k may be either one of the electro-capacitance C1 and C2, or difference of the electro-capacitance C1-C2, or, alternatively, electro-capacitance ratio C2/C1 in the above-described electro-capacitance sensor.
The above configuration of the present invention can be described as follows with an example of the electro-capacitance pressure sensor.
As shown in FIG. 1, an electro-capacitance pressure sensor 1 has a diaphragm 11 deforming in a direction orthogonal to the surface thereof by a pressure P to be detected, and a substrate 12 supporting the diaphragm 11 at the outer circumference thereof. The electro-capacitance pressure sensor 1 has a movable electrode 13 formed on a surface of the diaphragm 11 opposing the substrate 12, and first fixed electrode 14 and second fixed electrode 15 formed on the substrate 12 opposing the movable electrode 13, the electrodes 13 to 15 forming the sensor element. The movable electrode 13 follows the deformation of the diaphragm 11 in a direction orthogonal to the surface of the diaphragm 11, so that the movable electrode 13 approaches and recedes from the first fixed electrode 14 and the second fixed electrode 15 by the pressure P acting on the diaphragm 11. As shown in FIG. 2, the first fixed electrode 14 is formed on the substrate 12 in an approximate circle around deformation center of the diaphragm 11, and the second fixed electrode 15 is formed in a ring-shape surrounding outer circumference of the first fixed electrode 14. On the other hand, the movable electrode 13 is formed on the diaphragm 11 in an approximate circle corresponding to edge of the outer circumference of the second fixed electrode 15.
As shown in FIG. 3, the movable electrode 13 and the first fixed electrode 14, and the movable electrode 13 and the second fixed electrode 15 respectively form capacitors. When the electro-capacitance between the movable electrode 13 and the first fixed electrode 14 is C1, relationship shown in formula (8) generally stands true between the electro-capacitance C1 and the pressure P acting on the diaphragm 11. Incidentally, d is a distance between the movable electrode 13 and the first and the second fixed electrode 14 and 15, ∈S is relative permittivity of a clearance between the movable electrode 13 and the first fixed electrode 14, ∈0 is permittivity in vacuum, S1 is electrode area of the first fixed electrode 14 and A1 is displacement coefficient of the diaphragm 11 by the pressure P in the formula (8).                     C1        =                                            ϵ              S                        xc3x97                          ϵ              0                        xc3x97            S1                                d            -                          P              xc3x97              A1                                                          (        8        )            
Similarly, when the electro-capacitance between the movable electrode 13 and the second fixed electrode 15 is C2, relationship shown in formula (9) generally stands true between the electro-capacitance C2 and the pressure P. Incidentally, S2 is electrode area of the second fixed electrode 15 and A2 is displacement coefficient of the diaphragm 11 by the pressure P in the formula (9).                     C2        =                                            ϵ              S                        xc3x97                          ϵ              0                        xc3x97            S2                                d            -                          P              xc3x97              A2                                                          (        9        )            
Accordingly, the pressure P acting on the diaphragm 11 can be converted by detecting the electro-capacitance C1 and C2 from either one of the formulas (8) and (9).
However, it is known in the electro-capacitance pressure sensor 1 that the above-described electro-capacitance C1 and C2 cause error by temperature change and time change from initial electro-capacitance measurement value C10 and C20. Specifically, the electro-capacitance C1 and C2 at a certain time period has a relationship with the initial electro-capacitance measurement value C10 and C20 represented by the formulas (10) and (11).
Incidentally, the time change at a certain time period is represented by xcex94t and temperature change is represented by xcex94T as compared to the measurement period of the initial measurement value C10 and C20 in the formulas (10) and (11). Time change rate of the capacitor in accordance with the time change xcex94t and temperature change xcex94T is represented by m1 and m2, and temperature change rate is represented by n1 and n2 respectively.
C1=C10xc3x97(1+xcex94Txc3x97m1+xcex94txc3x97n1)xe2x80x83xe2x80x83(10)
C2=C20xc3x97(1+xcex94Txc3x97m2+xcex94txc3x97n2)xe2x80x83xe2x80x83(11)
Since the first fixed electrode 14 and the second fixed electrode 15 are made of the same component, the dielectric in the clearance between the movable electrode 13 and the first and the second fixed electrodes 14 and 15 can be considered common. That is, the capacitor for the electro-capacitance C1 and the capacitor for the electro-capacitance C2 may be considered to be configured of the same structure and the same components.
Accordingly, it is safely considered m1=m2 and n1=n2 in the formulas (10) and (11). Therefore, the error in accordance with the temperature change and the time change can be cancelled by detecting an electro-capacitance ratio k(=C2/C1) between the measured electro-capacitance C1 and the electro-capacitance C2, as shown in formula (12).                     k        =                              C2            C1                    =                                                    C2                0                                            C1                0                                      =                                                            ϵ                  S                                xc3x97                                  ϵ                  0                                xc3x97                S2                xc3x97                                  (                                      d                    -                                          P                      xc3x97                      A1                                                        )                                                                              ϵ                  S                                xc3x97                                  ϵ                  0                                xc3x97                S1                xc3x97                                  (                                      d                    -                                          P                      xc3x97                      A2                                                        )                                                                                        (        12        )            
Accordingly, the pressure P can be converted from the electro-capacitance ratio k by the formula (12) without considering the temperature change and the time change by detecting the electro-capacitance C1 and C2 (first correction).
The pressure P can be conducted from the electro-capacitance ratio k by assigning appropriate values to the coefficients xcex1, xcex2 and xcex3 in the following formula (13) based on the formula (12) (feedback correction). Incidentally, the formula (13) is identical with the above-described formula (6).                     P        =                              γ            xc3x97                          (                              k                -                β                            )                                            1            +                          α              xc3x97                              (                                  k                  -                  β                                )                                                                        (        13        )            
In the above formula (13), coefficient xcex1 is feedback gain, xcex3 is open loop gain and xcex2 is offset, each being calibration value for calibrating linearity, gain and offset of the sensor output respectively. The calibration of the electro-capacitance pressure sensor 1 is equal to calculating and storing the calibration value of the coefficients xcex1, xcex2 and xcex3.
The formula (13) stands true irrespective of the temperature change and time change after calculating the coefficients xcex1, xcex2 and xcex3. Accordingly, the pressure P can be accurately calculated by measuring the electro-capacitance ratio k and converting the electro-capacitance ratio k by the processor sing the formula (13).
The most appropriate calibration values of the coefficients xcex1, xcex2 and xcex3 are calculated by a coefficient arithmetic section of the processor and the calibration values are calculated by applying known calibration pressure to the electro-capacitance pressure sensor 1 for measuring the electro-capacitance ratio at the point.
Specifically, the electro-capacitance ratio k0 is calculated by measuring the electro-capacitance C1 and C2 under the condition P0(=0) without applying the pressure to the diaphragm 11 of the electro-capacitance pressure sensor 1. Subsequently, known calibration pressure P1 and P2 are applied to the electro-capacitance pressure sensor 1 and the electro-capacitance ratio k1 and k2 are calculated by measuring the electro-capacitance C1 and C2 at that point. Incidentally, the calibration pressure P1 and P2 are set as P2=2P1 for the convenience of following calculation.
P0 to P2 and k0 to k2 thus obtained are assigned to the formula (13). Accordingly, the calibration values of the coefficients xcex1, xcex2 and xcex3 can be calculated by solving the ternary simultaneous equations for the coefficients xcex1, xcex2 and xcex3 as shown in formulas (14) to (16).                     α        =                                            k              2                        -                          2              xc3x97                              k                1                                      +                          k              0                                                          (                                                k                  1                                -                                  k                  0                                            )                        +                          (                                                k                  2                                -                                  k                  0                                            )                                                          (        14        )                                β        =                  k          0                                    (        15        )                                          γ          =                                                                                          P                    2                                    xc3x97                                      (                                                                  k                        2                                            -                                              k                        1                                                              )                                                                                        (                                                                  k                        1                                            -                                              k                        0                                                              )                                    xc3x97                                      (                                                                  k                        2                                            -                                              k                        0                                                              )                                                              ⁢                              
                            ∵                              P                0                                      =            0                          ,                              P            2                    =                      2            xc3x97                          P              1                                                          (        16        )            
The calibration values of the coefficients xcex1, xcex2 and xcex3 are stored to E2PROM of non-volatile memory (not shown) as the calibration value of the electro-capacitance pressure sensor 1. The stored calibration values of xcex1, xcex2 and xcex3 are called by the calibration arithmetic section of the processor in measuring pressure by the electro-capacitance pressure sensor 1 and are assigned to the formula (13). After the electro-capacitance C1 and C2 are detected in accordance with the unknown measurement pressure, the calibration arithmetic section calculates the electro-capacitance ratio k and conducts calibration of the pressure P by the formula (13) to output.
Fundamental arithmetic section of the processor having the coefficient arithmetic section and the calibration arithmetic section calculates the above fundamental calculation formula (17) in response to predetermined arguments X, Y, Z and W from the coefficient arithmetic section and the calibration arithmetic section and outputs result f of the calculation to the coefficient arithmetic section or the calibration arithmetic section. Incidentally, the formula (17) is identical with the above-described formula (7).                     f        =                              Z            -            W                                X            -            Y                                              (        17        )            
Such fundamental arithmetic section works in calibrating for calculating the coefficients xcex1, xcex2 and xcex3, and in measuring unknown pressure by the electro-capacitance pressure sensor 1, as follows.
(1) Calculating Coefficients xcex1, xcex2 and xcex3 (Calibration of the Electro-Capacitance Pressure Sensor 1).
The calculation formula (14) of the coefficient xcex1 can be modified into following formula (18).                     α        =                                                            k                2                            -                              2                xc3x97                                  k                  1                                            +                              k                0                                                                    (                                                      k                    1                                    -                                      k                    0                                                  )                            xc3x97                              (                                                      k                    2                                    -                                      k                    0                                                  )                                              =                                                    (                                  1                  -                  0                                )                                            (                                                      k                    1                                    -                                      k                    0                                                  )                                      -                                          (                                  0                  -                  2                                )                                            (                                                      k                    2                                    -                                      k                    0                                                  )                                                                        (        18        )            
The coefficient arithmetic section outputs, for instance, k1, k0, 1 and 0 of the first term of the formula (18) as arguments X, Y, Z and W of the fundamental calculation formula (17). The fundamental arithmetic section conducts calculation given by the formula (17) and outputs the calculation result f to the coefficient arithmetic section. The calibration value of the coefficient xcex1 can be calculated to sequentially obtain the calculation result f by repeating similar process after the first term.
The calculation formula (15) of the coefficient xcex3 can be similarly modified into formula (19). The calibration value of the coefficient xcex3 can be calculated by repeating the calculation in the fundamental arithmetic section in plural times, similarly to the above.                     γ        =                                            (                                                P                  2                                -                0                            )                                      (                                                k                  1                                -                                  k                  0                                            )                                -                                    (                                                P                  2                                -                0                            )                                      (                                                k                  2                                -                                  k                  0                                            )                                                          (        19        )            
(2) Actual Pressure Measurement by the Electro-Capacitance Pressure Sensor 1
As described above, the calculated coefficients xcex1, xcex2 and xcex3 are stored in E2PROM as a non-volatile memory after completing the calibration of the electro-capacitance pressure sensor 1. The coefficients xcex1, xcex2 and xcex3 are called by the calibration arithmetic section in conducting measurement by the electro-capacitance sensor 1. The calibration arithmetic section detects the electro-capacitance C1 and C2, calculates the electro-capacitance ratio k (first correction) and converts the pressure P by the formula (13) to output (feedback correction). More specifically, the fundamental arithmetic section is used for the calculation and conversion as follows.
First, when the electro-capacitance ratio k is calculated by the electro-capacitance C1 and C2, the electro-capacitance ratio k(=C2/C1) can be modified into formula (20).                     k        =                              C2            C1                    =                                    C2              -              0                                      C1              -              0                                                          (        20        )            
In short, the calibration arithmetic section outputs C1, 0, C2 and 0 as the arguments X, Y, Z and W to the fundamental arithmetic section to obtain the calculation result f as the electro-capacitance ratio k.
Next, the formula (13) can be modified into following formula (21).                     P        =                                            γ              xc3x97                              (                                  k                  -                  β                                )                                                    1              +                              α                xc3x97                                  (                                      k                    -                    β                                    )                                                              =                                    γ              -              0                                                                        1                  -                  0                                                  (                                      k                    -                    β                                    )                                            -                                                α                  -                  0                                                  (                                      1                    -                    2                                    )                                                                                        (        21        )            
Accordingly, similarly to the above, the calibration arithmetic section can obtain the converted pressure P by making the fundamental arithmetic section repeatingly conduct calculation corresponding to the fundamental calculation formula (17) in the formula (21).
According to the present invention, since the processor has the fundamental arithmetic section for calculating the fundamental calculation formula (17), a part of the calculation by the coefficient arithmetic section and the calibration arithmetic section can be conducted by the fundamental arithmetic section, thereby simplifying the structure of the coefficient arithmetic section and calibration arithmetic section. Further, since the fundamental arithmetic section is used both for calibration of the sensor and the measurement, the processor can be made efficient by eliminating extra portion of the processor. Accordingly, the structure of the entire processor can be simplified, thereby reducing the total production cost including cost for calibration work of the self-calibrating sensor and component cost.
In the above, the processor is preferably provided on the sensor and composed of integrated circuit including CPU (Central Processing Unit). In other words, since the processor is provided on the sensor and is composed of integrated circuit including CPU, the sensor can be made as a self-calibrating sensor, thereby largely reducing the cost for the calibration work of the sensor, as described above.
Further, the above-described sensor preferably has a non-volatile memory of which recorded information is not lost even when the power supply from the outside is shut off, and the coefficients xcex1, xcex2 and xcex3 calculated by the coefficient arithmetic section are preferably stored in the non-volatile memory.
EPROM (Erasable Programmable Read Only Memory) of which recorded information can be erased by ultraviolet rays and E2PROM of which recorded information can be electrically erased are preferably used for the non-volatile memory. Since the sensor has such non-volatile memory, the coefficients xcex1, xcex2 and xcex3 are permanently stored in the memory after the calibration work is conducted in the self-calibrating sensor once, thereby eliminating the need for repeated calibration work in the subsequent use of the sensor.